1. Field of the Invention
The present invention relates to a golf ball of four or multilayer structure offering pleasant feel when hit, improved durability, and increased distance.
2. Prior Art
A variety of multi-piece golf balls including three-piece and four-piece ball constructions have been developed over the past few years in order to improve ball performance. The practice is known of balancing a soft feel with good resilience in multi-piece golf balls by giving the ball a hardness distribution across its respective layers (core, intermediate layer and cover) in such a way as to retain both properties. In particular, a number of patents have been described on techniques for softening the core in order to achieve a soft feel (see, for example, JP-B 4-55077, JP 2674627, and JP-A 7-194735).
The cores of the golf balls disclosed in these patents all have a diameter of about 30 mm, or at least about 20 mm. Softening the core significantly lowers its resilience, which must then be compensated for by increasing the hardness of the intermediate layer and cover, to provide a reasonable resilience for the ball as a whole. However, increasing the hardness of these layers gives the ball a poor feel. In addition, stress concentration due to differences in hardness arises at the interface between the soft core and the hard intermediate layer, causing the layers to separate. JP-A 11-417 discloses a core provided with an inner layer of relatively small diameter. Yet, here too, there exists a large difference in hardness between the inner layer and the intermediate layer that has been formed around and encloses the inner layer, resulting in interfacial adhesion problems such as interlayer separation, and poor durability due to rubber fissuring in the intermediate layer.